Re: [PATCH] mm: add a "struct page_frag" type containing a page, offset and length

[Ian Campbell <Ian.Campbell@citrix.com>]

On Fri, 2011-10-14 at 07:56 +0100, Ian Campbell wrote:
> On Thu, 2011-10-13 at 22:22 +0100, Andrew Morton wrote:
> > Looks OK to me.  I'm surprised we don't already have such a thing.
> > 
> > Review comments:
> > 
> > 
> > > +struct page_frag {
> > > +	struct page *page;
> > > +#if (BITS_PER_LONG > 32) || (PAGE_SIZE >= 65536)
> > 
> > It does add risk that people will add compile warnings and bugs by
> > failing to consider or test the other case.
> >
> > We could reduce that risk by doing
> > 
> >    #if (PAGE_SIZE >= 65536)
> > 
> > but then the 32-bit version would hardly ever be tested at all.
> 
> Indeed. The first variant has the benefit that most 32-bit arches will
> test one case and most 64-bit ones the other.
> 
> Perhaps the need to keep this struct small is not so acute as it is for
> the skb_frag_t I nicked it from and just using __u32 unconditionally is
> sufficient?

I wasn't sure what to do here so I left it as it was, so the only change
here is s/page_offset/offset/. Switching to a u32 only version is
something we can easily do in the future if this scheme turns out to be
problematic.

Ian.

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